High precision is desired in measurements of lengths or angles. Known measuring instruments used for these purposes, such as geodesic instruments, are designed in such a way that the operator reads the measured quantity from an analog scale usually formed by optical marks such as, for example, lines, gaps or numeric characters, or from a digital display. The technique of actually determining the measured quantity from the display device differs from one technique to another. The reading of scales is subjective but has the advantage of low apparatus cost. It is much more accurate to read from a digital display, but the devices necessary to produce an accurate digital display is normally somewhat more expensive. As is well known, geodesic measuring instruments such as, for example, theodolites, must have a small, simple and light-weight construction with limited power consumption. These instruments must also remain maintenance-free for many years and permit operation over that interval of time with the same reliable precision. Additionally, these instruments must be able to withstand very rough treatment since they are used in open country.
These requirements are not met by known digital measuring systems such as are described, for example, in German Offenlegungsschrift No. 2,211,235 and U.S. Pat. No. 3,973,119. It follows, for example, that static measuring systems have a precision which is not adequate for use as geodesic instruments. However, incremental measuring systems are sensitive to interruptions of the power supply because the angle or length value must be incrementally determined and stored as the value of the dimension changes. Finally, high-precision dynamic measuring systems are very complicated and costly and have drive and control systems which are sensitive to wear. Thus, these known measuring systems are expensive and complicated and must be frequently maintained and serviced during operation by highly paid technical individuals or else they are imprecise and, additionally, these systems do not have simple, reliable construction.